Binder for lithium-ion battery, aqueous binder solution for lithium-ion battery, electrode slurry for lithium-ion battery, electrode for lithium-ion battery, and lithium-ion battery

Abstract

This binder for a lithium-ion battery contains a water-soluble polymer (A). The water-soluble polymer (A) contains a structural unit derived from a hydroxyl-group-containing vinyl ether (a), and a structural unit derived from an ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof. The amount of the structural unit derived from the hydroxyl-group-containing vinyl ether (a) contained in the water-soluble polymer (A) is 5-80 mol% in all structural units of the water-soluble polymer (A), and the amount of the structural unit derived from the ethylenically unsaturated monomer (b) having the sulfonic acid group or a salt thereof in the water-soluble polymer (A) is 5-30 mol% in all structural units of the water-soluble polymer (A).

Description

Binder for lithium ion battery, aqueous binder solution for lithium ion battery, electrode slurry for lithium ion battery, electrode for lithium ion battery, and lithium ion battery

[0001] The present invention relates to a binder for a lithium ion battery, an aqueous binder solution for a lithium ion battery, an electrode slurry for a lithium ion battery, an electrode for a lithium ion battery, and a lithium ion battery.

[0002] Lithium-ion batteries are used in electronic devices such as mobile phones and laptops, and in recent years, they have also been increasingly used in automotive and home energy storage applications. Lithium-ion batteries consist of components such as a positive electrode, a negative electrode, a separator, and an electrolyte. The positive and negative electrodes are manufactured by applying electrode slurry, a mixture of an active material capable of inserting and extracting lithium ions, a conductive additive, a binder, a solvent, and other materials, onto a current collector and then drying the mixture.

[0003] Polyvinylidene fluoride (hereinafter also referred to as "PVDF") and the like have been widely used as binders for forming electrodes. However, PVDF needs to be dissolved in an organic solvent such as N-methylpyrrolidone, and an organic solvent has also been used as the liquid medium for electrode slurries. However, in recent years, from the viewpoints of environmental impact and cost, there has been an increasing demand for processes that use electrode slurries that use water as the liquid medium (hereinafter also referred to as "aqueous slurries"). For this reason, water-soluble binders that can be used in aqueous slurries have been developed.

[0004] Patent Document 1 discloses a binder used in an aqueous electrode composition for forming an electrode of a nonaqueous secondary battery, the binder containing a polymer having a structural unit (I) derived from an unsaturated polyalkylene glycol ether monomer (a).

[0005] JP 2011-119192 A

[0006] The technology of Patent Document 1 involves copolymerizing an unsaturated polyalkylene glycol ether monomer with an unsaturated carboxylic acid such as acrylic acid. The inclusion of structural units derived from an unsaturated carboxylic acid such as acrylic acid or its salt in a water-based polymer improves the dispersibility of the active material in an aqueous slurry. However, the inventors' studies have revealed that copolymerization of a monomer without an ester bond, such as an unsaturated polyalkylene glycol ether monomer, particularly a vinyl ether monomer, with an unsaturated carboxylic acid such as acrylic acid results in a high amount of residual monomer. A high amount of residual monomer can lead to a problem of not obtaining a polymer at the correct charge ratio. Furthermore, residual monomer can cause irreversible capacity and reduce coulombic efficiency, necessitating a separate removal process, which reduces productivity. On the other hand, reducing the amount of vinyl ether monomer used to reduce the amount of residual monomer can lead to the problem of cracking in the active material layer during electrode formation. Therefore, when structural units derived from vinyl ether monomers are included, it has been difficult to obtain a binder capable of forming electrodes while suppressing the generation of residual monomer.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a lithium ion battery binder capable of forming an electrode containing a water-soluble polymer that contains structural units derived from vinyl ether monomers and in which the generation of residual monomers during synthesis is suppressed; an aqueous lithium ion battery binder solution that uses the lithium ion battery binder; an electrode slurry for lithium ion batteries; an electrode for lithium ion batteries; and a lithium ion battery.

[0008] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by a water-soluble polymer containing structural units derived from a specific monomer, and have completed the present invention as described below. That is, the present invention relates to the following items [1] to [9]. [1] A binder for lithium ion batteries containing a water-soluble polymer (A), wherein the water-soluble polymer (A) contains structural units derived from a hydroxyl group-containing vinyl ether (a) and structural units derived from an ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof, the content of the structural units derived from the hydroxyl group-containing vinyl ether (a) in the water-soluble polymer (A) being 5 to 80 mol % of all structural units of the water-soluble polymer (A), and the content of the structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof in the water-soluble polymer (A) being 5 to 30 mol % of all structural units of the water-soluble polymer (A). [2] The binder for lithium ion batteries according to [1] above, wherein the water-soluble polymer (A) further contains structural units derived from a (meth)acrylamide group-containing compound (c) in an amount of 20 to 85 mol % of all structural units of the water-soluble polymer (A). [3] The binder for lithium ion batteries according to [1] or [2] above, wherein the hydroxyl group-containing vinyl ether (a) is a vinyl ether having a hydroxyalkyl group having 1 to 3 carbon atoms, or a vinyl ether having a hydroxypolyoxyalkylene chain containing two or more oxyalkylene units having 1 to 3 carbon atoms. [4] The binder for lithium ion batteries according to any one of [1] to [3] above, wherein the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof is a compound having a sulfonic acid group or a salt thereof and a (meth)acryloyl group. [5] An aqueous binder solution for lithium ion batteries containing the binder for lithium ion batteries according to any one of [1] to [4] above and water. [6] The aqueous binder solution for lithium ion batteries according to the above [5], wherein the total content of the water-soluble polymer (A) and water is 80 to 100 mass % of the total amount of the aqueous binder solution for lithium ion batteries. [7] An electrode slurry for lithium ion batteries, comprising the binder for lithium ion batteries according to any one of the above [1] to [4], an active material, and water.[8] A lithium ion battery electrode containing the lithium ion battery binder and active material according to any one of [1] to [4] above. [9] A lithium ion battery having the lithium ion battery electrode according to [8] above.

[0009] According to the present invention, it is possible to provide a lithium ion battery binder capable of forming an electrode containing a water-soluble polymer that contains a structural unit derived from a vinyl ether monomer and in which the generation of residual monomers during synthesis is suppressed; an aqueous lithium ion battery binder solution that uses the lithium ion battery binder; an electrode slurry for a lithium ion battery; an electrode for a lithium ion battery; and a lithium ion battery.

[0010] In this specification, the lower and upper limits of preferred numerical ranges described in stages can be independently combined. For example, the description "preferably 10 to 90, more preferably 30 to 60" can be combined with the "preferable lower limit (10)" and the "more preferable upper limit (60)" to form "10 to 60."

[0011] In this specification, for example, "(meth)acrylic acid" refers to both "acrylic acid" and "methacrylic acid," and the same applies to other similar terms.

[0012] In this specification, the term "structural unit" refers to a structural unit based on one monomer molecule, which is formed by polymerizing one monomer molecule.

[0013] [Binder for Lithium-ion Batteries] The binder for lithium-ion batteries of the present embodiment (hereinafter also simply referred to as "the binder of the present embodiment") is a binder for lithium-ion batteries containing a water-soluble polymer (A), wherein the water-soluble polymer (A) contains structural units derived from a hydroxyl group-containing vinyl ether (a) and structural units derived from an ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof, the content of the structural units derived from the hydroxyl group-containing vinyl ether (a) in the water-soluble polymer (A) is 5 to 80 mol % of all structural units of the water-soluble polymer (A), and the content of the structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof in the water-soluble polymer (A) is 5 to 30 mol % of all structural units of the water-soluble polymer (A).

[0014] <Water-soluble polymer (A)> The water-soluble polymer (A) contains a structural unit derived from a hydroxyl group-containing vinyl ether (a) and a structural unit derived from an ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof. In this specification, "vinyl ether" refers to a compound having a vinyl group and an etheric oxygen atom bonded to the vinyl group. In this specification, "ethylenically unsaturated monomer" refers to a monomer having an ethylenically unsaturated group. In addition, the "water-soluble" in the water-soluble polymer (A) means that the solubility in 100 parts by mass of water at 23°C is 1 part by mass or more.

[0015] The water-soluble polymer (A) contains structural units derived from the hydroxyl group-containing vinyl ether (a), which gives it excellent hydrolysis resistance in an alkaline environment. Furthermore, the water-soluble polymer (A) contains structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof, which suppresses the generation of residual monomers during the synthesis of the water-soluble polymer (A). While the reason for this is unclear, it is presumed that one factor is that the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof promotes the polymerization reaction of the hydroxyl group-containing vinyl ether (a).

[0016] (Structural Units Derived from Hydroxyl-Containing Vinyl Ether (a)) The structural units derived from the hydroxyl-containing vinyl ether (a) are structural units formed by an addition reaction of the vinyl groups possessed by the hydroxyl-containing vinyl ether (a). The structural units derived from the hydroxyl-containing vinyl ether (a) contained in the water-soluble polymer (A) may be of one type alone or may be of two or more types.

[0017] [Hydroxyl group-containing vinyl ether (a)] The hydroxyl group-containing vinyl ether (a) is not particularly limited as long as it is a compound having a hydroxyl group, a vinyl group, and an etheric oxygen atom bonded to the vinyl group. The number of vinyl groups that the hydroxyl group-containing vinyl ether (a) has in one molecule is one or more, and from the viewpoint of suppressing gelation during synthesis, it is preferably three or less, more preferably two or less, and even more preferably one.

[0018] From the viewpoints of reactivity, flexibility, etc., the hydroxyl group-containing vinyl ether (a) is preferably a vinyl ether having a hydroxyalkyl group having 1 to 3 carbon atoms or a vinyl ether having a hydroxypolyoxyalkylene chain containing two or more oxyalkylene units having 1 to 3 carbon atoms.

[0019] Examples of the hydroxyalkyl group having 1 to 3 carbon atoms contained in the vinyl ether having a hydroxyalkyl group having 1 to 3 carbon atoms include a hydroxymethyl group, a 2-hydroxyethyl group, and a 3-hydroxypropyl group. Examples of the vinyl ether having a hydroxyalkyl group having 1 to 3 carbon atoms include hydroxyalkyl vinyl ethers such as hydroxymethyl vinyl ether, 2-hydroxyethyl vinyl ether, and 3-hydroxypropyl vinyl ether. Of these, 2-hydroxyethyl vinyl ether is preferred.

[0020] Examples of the oxyalkylene units having 1 to 3 carbon atoms contained in the vinyl ether having a hydroxypolyoxyalkylene chain containing two or more oxyalkylene units having 1 to 3 carbon atoms include an oxymethylene unit, an oxyethylene unit, and an oxypropylene unit. The number of oxyalkylene units in the hydroxypolyoxyalkylene chain is two or more, and preferably four or less, more preferably three or less, and even more preferably two. Examples of vinyl ethers having a hydroxypolyoxyalkylene chain containing two or more oxyalkylene units having 1 to 3 carbon atoms include diethylene glycol monovinyl ether, triethylene glycol monovinyl ether, dipropylene glycol monovinyl ether, and tripropylene glycol monovinyl ether. Among these, diethylene glycol monovinyl ether is preferred.

[0021] [Content of structural units derived from hydroxyl group-containing vinyl ether (a)] The content of structural units derived from hydroxyl group-containing vinyl ether (a) in the water-soluble polymer (A) is 5 to 80 mol% of the total structural units of the water-soluble polymer (A). When the content of structural units derived from hydroxyl group-containing vinyl ether (a) is 5 mol% or more, the polymer becomes flexible, and when used as a binder, electrode cracking can be suppressed and the hydrolysis resistance of the water-soluble polymer (A) can be improved. Furthermore, when the content of structural units derived from hydroxyl group-containing vinyl ether (a) is 80 mol% or less, the generation of residual monomers during the synthesis of the water-soluble polymer (A) can be suppressed. From the same viewpoint as above, the content of structural units derived from hydroxyl group-containing vinyl ether (a) in the water-soluble polymer (A) is preferably 10 to 78 mol%, more preferably 20 to 75 mol%, and even more preferably 30 to 73 mol% of the total structural units of the water-soluble polymer (A).

[0022] (Structural Unit Derived from Ethylenically Unsaturated Monomer (b) Having a Sulfonic Acid Group or a Salt Thereof) The structural unit derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof is a structural unit formed by an addition reaction of the ethylenically unsaturated group possessed by the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof. In this specification, the term "ethylenically unsaturated group" refers to a functional group containing an ethylenically unsaturated bond. The structural unit derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof may be of one type alone or of two or more types.

[0023] [Ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof] The ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof has a sulfonic acid group or a salt thereof and an ethylenically unsaturated group. Examples of the ethylenically unsaturated group possessed by the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof include a vinyl group, an allyl group, and a (meth)acryloyl group. Among these, a (meth)acryloyl group is preferred. That is, the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof is preferably a compound having a sulfonic acid group or a salt thereof and a (meth)acryloyl group.

[0024] The salt of a sulfonic acid group is a salt of a sulfonic acid group (-S(=O) ２ For example, the sodium salt of a sulfonic acid group (-S(=O) ２ ONa), potassium salt of sulfonic acid group (-S(=O) ２ OK), ammonium salt of sulfonic acid group (-S(=O) ２ ONH ４ ) etc.

[0025] Examples of the compound having a sulfonic acid group or a salt thereof and a (meth)acryloyl group include (meth)acrylamides having a sulfonic acid group or a salt thereof, and specifically, 2-acrylamido-2-methyl-1-propanesulfonic acid or a sodium salt thereof is preferred.

[0026] [Content of Structural Units Derived from Ethylenically Unsaturated Monomer (b) Having a Sulfonic Acid Group or a Salt Thereof] The content of structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof in the water-soluble polymer (A) of this embodiment is 5 to 30 mol % of the total structural units of the water-soluble polymer (A). When the content of structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof is 5 mol % or more, the dispersibility of the active material is improved and the generation of residual monomers during the synthesis of the water-soluble polymer (A) can be suppressed. When the content of structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof is 30 mol % or less, the water absorption rate of the water-soluble polymer can be suppressed. From the same viewpoint as above, the content of structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof in the water-soluble polymer (A) of this embodiment is preferably 5.1 to 29 mol %, more preferably 5.3 to 20 mol %, and even more preferably 5.5 to 10 mol % of the total structural units of the water-soluble polymer (A).

[0027] (Structural Units Derived from (Meth)acrylamide Group-Containing Compound (c)) The water-soluble polymer (A) preferably further contains structural units derived from a (meth)acrylamide group-containing compound (c). However, in the present invention, a (meth)acrylamide group-containing compound having a sulfonic acid group or a salt thereof is classified as an ethylenically unsaturated monomer having a sulfonic acid group or a salt thereof (b).

[0028] The structural unit derived from the (meth)acrylamide group-containing compound (c) is a structural unit formed by an addition reaction of a carbon-carbon double bond contained in a (meth)acryloyl group of the (meth)acrylamide group-containing compound (c). When the water-soluble polymer (A) contains a structural unit derived from the (meth)acrylamide group-containing compound (c), the reactivity between monomers during synthesis of the water-soluble polymer (A) tends to be improved, and the generation of residual monomers can be further suppressed. In this specification, the "(meth)acrylamide group" refers to a group represented by the following general formula (c-1):

[0029] (In the formula, Rｃ１ indicates a hydrogen atom or a methyl group. * indicates a bonding site.)

[0030] The structural unit derived from the (meth)acrylamide group-containing compound (c) contained in the water-soluble polymer (A) may be of one type alone or may be of two or more types.

[0031] [(Meth)acrylamide Group-Containing Compound (c)] The (meth)acrylamide group-containing compound (c) is not particularly limited as long as it is a compound containing the above-mentioned (meth)acrylamide group, and examples thereof include compounds represented by the following general formula (c-2):

[0032] (In the formula, R ｃ１ represents a hydrogen atom or a methyl group. ｃ２ and R ｃ３ each independently represents a hydrogen atom or a substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms; R ｃ２ and R ｃ３ may be bonded to form a ring.)

[0033] In the above general formula (c-2), R ｃ２ and R ｃ３ Examples of the substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms represented by R include a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, a substituted or unsubstituted alkenyl group having 2 to 20 carbon atoms, a substituted or unsubstituted alkynyl group having 2 to 20 carbon atoms, and a substituted or unsubstituted aromatic hydrocarbon group having 6 to 20 carbon atoms. ｃ２ and R ｃ３ The ring formed by bonding with R ｃ２ and R ｃ３ a heterocyclic ring formed from a nitrogen atom to which R is bonded and 2 to 20 carbon atoms; ｃ２ and R ｃ３and heterocycles formed from a nitrogen atom to which the group bonded, one or more oxygen atoms, and 2 to 20 carbon atoms. The number of carbon atoms in the substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms may be 1 to 10 or 1 to 5. Note that the number of carbon atoms does not include the number of carbon atoms in the substituent. Examples of substituents that may be contained in the substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms include a hydroxy group, a halogen atom, a cyano group, a nitro group, a carboxy group, an amino group, an alkoxy group, an aryloxy group, and an acyloxy group. The number of carbon atoms in the substituent may be 1 to 5 or 1 to 3.

[0034] Among the above options, R ｃ２ and R ｃ３ is preferably a hydrogen atom from the viewpoint of reactivity.

[0035] Examples of the (meth)acrylamide group-containing compound (c) include (meth)acrylamide; N-alkyl(meth)acrylamides such as N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-n-butyl(meth)acrylamide, N-t-butyl(meth)acrylamide, and N-hexyl(meth)acrylamide; N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide; (meth)acrylamides having a hydroxyalkyl group such as N-hydroxymethyl(meth)acrylamide, N-(2-hydroxyethyl)(meth)acrylamide, and N-(2-hydroxypropyl)(meth)acrylamide; and (meth)acrylamides having a ring structure such as N-acryloylpyrrolidine, 3-acryloyl-2-oxazolidinone, 4-acryloylmorpholine, N-acryloylpiperidine, and N-methacryloylpiperidine. Among these, (meth)acrylamide is preferred from the viewpoint of reactivity.

[0036] [Content of Structural Units Derived from (Meth)acrylamide Group-Containing Compound (c)] When the water-soluble polymer (A) of this embodiment contains structural units derived from the (meth)acrylamide group-containing compound (c), the content of the structural units derived from the (meth)acrylamide group-containing compound (c) in the water-soluble polymer (A) is preferably 10 to 90 mol%, more preferably 30 to 80 mol%, even more preferably 40 to 75 mol%, even more preferably 50 to 70 mol%, and particularly preferably 55 to 60 mol%, based on the total structural units of the water-soluble polymer (A). When the content of the structural units derived from the (meth)acrylamide group-containing compound (c) is equal to or greater than the above-mentioned lower limit, the generation of residual monomers during the synthesis of the water-soluble polymer (A) tends to be more effectively suppressed. Furthermore, when the content of the structural units derived from the (meth)acrylamide group-containing compound (c) is equal to or less than the above-mentioned upper limit, curling during electrode drying tends to be more effectively suppressed.

[0037] (Other Structural Units) The water-soluble polymer (A) of this embodiment may or may not further contain structural units (hereinafter also referred to as "other structural units") derived from other monomers other than the hydroxyl group-containing vinyl ether (a), the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof, and the (meth)acrylamide group-containing compound (c). Examples of the other monomers include α-olefin compounds such as ethylene, propylene, n-butene, and isobutylene; (meth)acrylic acid ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, and n-butyl (meth)acrylate; unsaturated carboxylic acid compounds or salts thereof such as acrylic acid, methacrylic acid, and sodium acrylate; and vinyl cyanide compounds such as acrylonitrile and methacrylonitrile. The other structural units contained in the water-soluble polymer (A) may be of one type alone, or may be of two or more types.

[0038] When the water-soluble polymer (A) of the present embodiment contains other structural units, the content of the other structural units in the water-soluble polymer (A) is preferably 60 mol % or less, more preferably 40 mol % or less, and even more preferably 20 mol % or less, of the total structural units of the water-soluble polymer (A).

[0039] (Total Content of Structural Units Derived from Hydroxyl Group-Containing Vinyl Ether (a), Structural Units Derived from Ethylenically Unsaturated Monomer (b) Having a Sulfonic Acid Group or a Salt Thereof, and Structural Units Derived from (Meth)acrylamide Group-Containing Compound (c)) The total content of the structural units derived from the hydroxyl group-containing vinyl ether (a), the structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof, and the structural units derived from the (meth)acrylamide group-containing compound (c) in the water-soluble polymer (A) of the present embodiment is preferably 40 to 100 mol %, more preferably 60 to 100 mol %, and even more preferably 80 to 100 mol %, of all structural units in the water-soluble polymer (A).

[0040] The residual monomer content of the water-soluble polymer (A) is preferably 9.00 mol% or less, more preferably 6.00 mol% or less, even more preferably 3.00 mol% or less, and even more preferably 1.00 mol% or less. The residual monomer content of the water-soluble polymer (A) can be measured by the method described in the Examples.

[0041] (Method for producing water-soluble polymer (A)) The water-soluble polymer (A) can be produced by polymerizing raw material monomers containing a hydroxyl group-containing vinyl ether (a), an ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof, and a (meth)acrylamide group-containing compound (c) used as needed, other monomers, etc., in a solvent. The polymerization method is not particularly limited, and may be, for example, any method such as solution polymerization or bulk polymerization. From the viewpoint of productivity, solution polymerization is preferred, and solution polymerization using water as a solvent is more preferred.

[0042] When carrying out the polymerization reaction, it is preferable to use a polymerization initiator. Examples of the polymerization initiator include azo compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2-methylpropionamidine) dihydrochloride, and 2,2'-azobis(2-hydroxymethylpropionitrile); organic peroxides such as lauryl peroxide, tert-butyl hydroperoxide, benzoyl peroxide, tert-butyl peroxybenzoate, and (3,5,5-trimethylhexanoyl)peroxide; and inorganic peroxides such as potassium persulfate, ammonium persulfate, and hydrogen peroxide. Among these, azo compounds are preferred, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride is more preferred. One type of polymerization initiator may be used alone, or two or more types may be used in combination.

[0043] The amount of the polymerization initiator used is preferably 0.01 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, and even more preferably 0.5 to 3 parts by mass, based on the total amount of monomers (100 parts by mass).

[0044] The polymerization reaction is preferably carried out under heating. The reaction temperature is, for example, 50 to 95°C, and the reaction time is, for example, 2 to 8 hours. The monomers may be added all at once or may be added gradually. The polymerization reaction is preferably carried out under an inert atmosphere such as nitrogen with stirring.

[0045] <Components Other Than Water-Soluble Polymer (A)> The binder for lithium ion batteries of this embodiment may or may not contain components other than the water-soluble polymer (A) as necessary. Examples of components other than the water-soluble polymer (A) include polymers other than the water-soluble polymer (A). The content of the water-soluble polymer (A) in the binder for lithium ion batteries of this embodiment is preferably 90 to 100% by mass, more preferably 95 to 100% by mass, and even more preferably 99 to 100% by mass. The binder for lithium ion batteries of this embodiment may be the water-soluble polymer (A) itself.

[0046] <Use of the binder for lithium ion batteries according to this embodiment> The binder according to this embodiment is used to form an electrode containing an active material. The electrode formed using the binder according to this embodiment may be a positive electrode or a negative electrode. The active material used in combination with the binder according to this embodiment is not particularly limited and can be appropriately selected from known active materials. Specific examples of the active material used in combination with the binder according to this embodiment are as exemplified in the "electrode slurry for lithium ion batteries" section described below.

[0047] [Aqueous binder solution for lithium ion batteries] The aqueous binder solution for lithium ion batteries of the present embodiment (hereinafter also simply referred to as "aqueous binder solution of the present embodiment") is an aqueous binder solution for lithium ion batteries containing the binder for lithium ion batteries of the present embodiment and water. An electrode formed using the aqueous binder solution of the present embodiment may be a positive electrode or a negative electrode.

[0048] The content of the water-soluble polymer (A) in the aqueous binder solution of the present embodiment is preferably 1 to 40% by mass, more preferably 5 to 30% by mass, and even more preferably 10 to 20% by mass, from the viewpoint of ease of handling.

[0049] The binder aqueous solution of this embodiment may or may not contain other components (hereinafter also referred to as "other aqueous solution components") other than the binder of this embodiment and water. Examples of other aqueous solution components include solvents other than water, thickeners, humectants, pH adjusters, stabilizers, surfactants, antioxidants, preservatives, etc. Each of these may be used alone, or two or more may be used in combination.

[0050] From the viewpoint of more fully exhibiting the effects of the present invention, the total content of the water-soluble polymer (A) and water is preferably 80 to 100 mass %, more preferably 90 to 100 mass %, and even more preferably 95 to 100 mass %, of the total amount of the aqueous binder solution of the present embodiment.

[0051] The method for producing the aqueous binder solution of this embodiment is not particularly limited. For example, when the water-soluble polymer (A) is synthesized using water as a solvent to obtain the aqueous solution, the aqueous solution of the water-soluble polymer (A) may be used as is as the aqueous binder solution of this embodiment. Furthermore, if necessary, the aqueous binder solution may be produced by isolating the water-soluble polymer (A) synthesized in a solvent and then dissolving it in water at a desired time. Other aqueous solution components may be added at any time.

[0052] [Electrode Slurry for Lithium-Ion Batteries] The electrode slurry for lithium-ion batteries of this embodiment (hereinafter also simply referred to as "electrode slurry of this embodiment") is an electrode slurry for lithium-ion batteries containing the binder for lithium-ion batteries of this embodiment, an active material, and water. The electrode slurry of this embodiment is applied to a current collector and then dried, and is used to form an active material layer containing the active material and the binder of this embodiment on the current collector. The electrode formed using the electrode slurry of this embodiment may be a positive electrode or a negative electrode.

[0053] The active material contained in the electrode slurry of this embodiment is not particularly limited, and can be appropriately selected from known active materials for positive electrodes and active materials for negative electrodes according to the purpose.

[0054] Examples of the active material for the positive electrode include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium iron phosphate, ternary positive electrode materials, and quaternary positive electrode materials. Examples of the ternary positive electrode materials include LiNi １／３ Co １／３ Mn １／３ O ２ , LiNi ０．５ Co ０．３ Mn ０．２ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．６ Co ０．２ Mn ０．２ O ２ , LiNi ０．８ Co ０．１ Mn０．１ O ２ LiNi etc. ｘ Co ｙ Mn ｚ O ２ Nickel-cobalt-manganese ternary cathode material (NCM) represented by (0<x<1, 0<y<1, 0<z<1, and x+y+z=1), LiNi ０．８ Co ０．１５ Al ０．０５ O ２ LiNi etc. ｘ Co ｙ Al ｚ O ２ (0<x<1, 0<y<1, 0<z<1, and x+y+z=1), and nickel-cobalt-aluminum ternary cathode materials (NCA) are examples. The active material for the positive electrode may be used alone or in combination of two or more. The average particle diameter (D ５０ ) is not particularly limited and may be, for example, 0.01 to 30 μm, 0.1 to 20 μm, or 1 to 15 μm.

[0055] Examples of the active material for the negative electrode include graphite, hard carbon, lithium titanate, Si, SiO, and lithium-doped SiO. The active material for the negative electrode may be used alone or in combination of two or more. The average particle diameter (D ５０ ) is not particularly limited and may be, for example, 0.1 to 50 μm, 0.5 to 45 μm, or 1 to 30 μm.

[0056] The shape of the active material is not particularly limited, and may be, for example, crushed, spherical, flat, fibrous, or the like.

[0057] Here, the water-soluble polymer (A) contained in the binder of this embodiment has excellent hydrolysis resistance in an alkaline environment due to the structural unit derived from the hydroxyl group-containing vinyl ether (a). Therefore, by using the binder of this embodiment in combination with an active material that exhibits alkalinity in an aqueous slurry state, electrode production can be performed more stably than with aqueous slurries using conventional water-soluble binders. From this perspective, the active material contained in the electrode slurry of this embodiment is preferably an active material that exhibits a high pH in an aqueous slurry state. Examples of active materials that exhibit a high pH in an aqueous slurry state include lithium-doped SiO and ternary positive electrode materials.

[0058] In the electrode slurry of this embodiment, the content of the binder of this embodiment relative to 100 parts by mass of the active material is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 8 parts by mass, and even more preferably 1 to 5 parts by mass. When the content of the binder of this embodiment is equal to or greater than the above-mentioned lower limit, the active material tends to be more firmly bound. On the other hand, when the content of the binder of this embodiment is equal to or less than the above-mentioned upper limit, an increase in the resistance of the electrode tends to be suppressed.

[0059] The electrode slurry of this embodiment may contain a conductive aid as needed. Examples of conductive aids include various graphites; carbon blacks such as acetylene black, ketjen black, furnace black, and channel black; carbon fibers such as carbon nanofibers and carbon nanotubes; and metal materials such as nickel, aluminum, and stainless steel. The shape of the conductive aid is not particularly limited and may be particulate or fibrous. One type of conductive aid may be used alone, or two or more types may be used in combination. When the electrode slurry of this embodiment contains a conductive aid, the content of the conductive aid relative to 100 parts by mass of the active material is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 8 parts by mass, and even more preferably 0.1 to 5 parts by mass.

[0060] From the viewpoint of coatability, the solid content concentration in the electrode slurry of this embodiment is preferably 20 to 90 mass %, more preferably 25 to 85 mass %, and even more preferably 30 to 80 mass %. In this specification, the solid content of the electrode slurry means components other than the liquid medium.

[0061] The electrode slurry of this embodiment may or may not contain components other than the binder of this embodiment, the active material, and water, as well as the conductive additive used as needed (hereinafter also referred to as "other slurry components"). Examples of the other slurry components include the same as the other aqueous solution components described above.

[0062] The method for producing the electrode slurry of this embodiment is not particularly limited, and the electrode slurry can be produced, for example, by mixing the binder of this embodiment or the aqueous binder solution of this embodiment, the active material, the conductive additive, a solvent such as water, and other slurry components, which are used as needed, by a known method. Since the binder of this embodiment is one in which the generation of residual monomers is suppressed, an aqueous solution of the water-soluble polymer (A) containing water as a reaction solvent when synthesizing the water-soluble polymer (A) can be used directly to produce the electrode slurry. Examples of the mixing device that can be used include kneaders such as a ball mill, a sand mill, a planetary mixer, a paint shaker, a homomixer, a homodisper, and a homogenizer.

[0063] [Electrode for Lithium Ion Battery] The electrode for a lithium ion battery of this embodiment (hereinafter also simply referred to as "electrode of this embodiment") is an electrode for a lithium ion battery containing the binder for a lithium ion battery of this embodiment and an active material. The electrode of this embodiment may be a positive electrode or a negative electrode. The electrode of this embodiment preferably has an active material layer containing an active material and the binder of this embodiment on a current collector. In the electrode of this embodiment, the thickness of the active material layer is not particularly limited and is, for example, 100 to 300 μm.

[0064] The description of the active material contained in the electrode of this embodiment is the same as the description of the active material contained in the electrode slurry of this embodiment described above. In the electrode of this embodiment, the content of the binder of this embodiment relative to 100 parts by mass of active material is the same as the preferred range described in the description of the electrode slurry of this embodiment. In addition, the electrode of this embodiment may contain the above-mentioned conductive auxiliary agent. In the electrode of this embodiment, the content of the conductive auxiliary agent relative to 100 parts by mass of active material is the same as the preferred range described in the description of the electrode slurry of this embodiment. In addition, the electrode of this embodiment may contain components exemplified as the above-mentioned other slurry components and other aqueous solution components.

[0065] The current collector is preferably made of a sheet-like metal. Examples of metals used for the current collector include iron, copper, aluminum, nickel, and stainless steel. Among these, aluminum foil is preferred as the current collector for the positive electrode, and copper foil is preferred as the current collector for the negative electrode. The thickness of the current collector is, for example, 5 to 20 μm.

[0066] The electrode of this embodiment can be produced by applying the electrode slurry of this embodiment onto a current collector and then drying it. Examples of methods for applying the electrode slurry of this embodiment onto a current collector include a reverse roll method, a doctor blade method, a direct roll method, a knife method, an extrusion method, a curtain method, a gravure method, a bar method, a dip method, and a squeeze method.

[0067] The electrode slurry of this embodiment may be applied to only one surface of the current collector, or may be applied to both surfaces. When the electrode slurry is applied to both surfaces of the current collector, it may be applied to one surface at a time, or to both surfaces simultaneously. Furthermore, the electrode slurry may be applied continuously to the entire surface of the current collector, or may be applied intermittently. The amount and application range of the electrode slurry may be determined appropriately depending on the size of the battery, etc.

[0068] Examples of methods for drying the electrode slurry applied to the current collector include a method of applying hot air, a vacuum drying method, a method using electromagnetic waves such as an infrared, far-infrared, or near-infrared heater, etc. The drying temperature for the electrode slurry is, for example, 80 to 150°C, and the drying time is, for example, 1 to 30 minutes.

[0069] The obtained electrode may be subjected to shaping such as cutting, if necessary.

[0070] [Lithium-ion battery] The lithium-ion battery of this embodiment is a lithium-ion battery having the lithium-ion battery electrode of this embodiment. Examples of the lithium-ion battery of this embodiment include a lithium-ion secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator. In the lithium-ion battery of this embodiment, at least one of the positive electrode and the negative electrode includes the electrode of this embodiment, and so long as this is the case, the battery may include an electrode other than the electrode of this embodiment. The electrode other than the electrode of this embodiment, the electrolyte, and the separator may be appropriately selected from known types depending on the application. The shape of the lithium-ion battery of this embodiment is not particularly limited, and may be any shape, such as a coin shape, a button shape, a sheet shape, a cylindrical shape, a prismatic shape, or a flat shape.

[0071] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0072] [Preparation of Aqueous Binder Solution for Lithium-Ion Batteries] Example 1: A reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube was charged with 1,274.8 g of purified water, 200.0 g of diethylene glycol monovinyl ether, 163.0 g of an aqueous solution of 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (concentration: 50% by mass), and 160.0 g of an aqueous acrylamide solution (concentration: 50% by mass). The temperature was raised to 55°C, and oxygen was removed from the reaction system by introducing nitrogen gas. A solution containing 2.4 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 24.0 g of purified water was added, and the temperature was raised to 80°C, allowing the reaction to proceed for 4 hours. Subsequently, 1,106.6 g of purified water was added to obtain an aqueous solution of water-soluble polymer (A). The resulting aqueous solution of water-soluble polymer (A) was used as an aqueous binder solution for lithium-ion batteries (water-soluble polymer (A) content: 13% by mass).

[0073] Example 2: 743.2 g of purified water, 200.0 g of 2-hydroxyethyl vinyl ether, and 200.0 g of an aqueous solution of 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (concentration: 50% by mass) were added to a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube. The temperature was raised to 55°C, and oxygen was removed from the reaction system by passing nitrogen gas through the reactor. A solution containing 2.4 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 24.0 g of purified water was added to the reactor, and the temperature was raised to 80°C, allowing the reaction to proceed for 4 hours. Subsequently, 1712.5 g of purified water was added to obtain an aqueous solution of water-soluble polymer (A). The resulting aqueous solution of water-soluble polymer (A) was used as an aqueous binder solution for lithium-ion batteries (water-soluble polymer (A) content: 13% by mass).

[0074] Comparative Example 1 An aqueous solution of a water-soluble polymer was prepared in the same manner as in Example 1, except that 163.0 g of an aqueous solution of sodium acrylate (concentration: 32% by mass) was used instead of 163.0 g of the aqueous solution of 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (concentration: 50% by mass) in Example 1. The obtained aqueous solution of the water-soluble polymer was used as an aqueous binder solution for lithium ion batteries (water-soluble polymer content: 13% by mass).

[0075] Comparative Example 2: 1805.53 g of purified water, 25.00 g of diethylene glycol monovinyl ether, 644.64 g of a 50% by weight aqueous acrylamide solution, and 0.38 g of sodium methallylsulfonate were added to a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube. The temperature was raised to 55°C, and oxygen was removed from the reaction system by passing nitrogen gas through the reactor. A solution containing 3.48 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 41.72 g of purified water was added, and the temperature was raised to 80°C, allowing the reaction to proceed for 4 hours. Subsequently, 180.63 g of purified water was added to obtain an aqueous solution of water-soluble polymer. This aqueous solution of water-soluble polymer was used as an aqueous binder solution for lithium-ion batteries (water-soluble polymer content: 13% by weight).

[0076] Comparative Example 3 792.80 g of purified water and 200.00 g of diethylene glycol monovinyl ether were added to a reaction apparatus equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, and the temperature was raised to 55°C. Oxygen was removed from the reaction system by passing nitrogen gas through the system. A solution containing 2.00 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 20.00 g of purified water was added to the reaction system, and the temperature was raised to 80°C, allowing the reaction to proceed for 4 hours. Thereafter, 545.05 g of purified water was added, and the reaction solution was １ When H-NMR measurement was carried out, only the peak of the monomer was confirmed, and it was therefore determined that polymerization had not progressed.

[0077] [Production of Lithium-ion Battery Electrode Slurry and Lithium-ion Battery Electrode] 13.6 g of the aqueous binder solution for lithium-ion batteries obtained in each Example and Comparative Example and carbon black (manufactured by MTI Corporation, trade name "Super C65", specific surface area by BET method: 62 m) as a conductive aid were placed in a glass container (volume: 140 mL). ２ 0.84 g of cellulose acetate (0.84 g / g) and 41.4 g of purified water were added, and the mixture was stirred at 2,000 rpm for 30 minutes using a disperser (manufactured by Primix Corporation, trade name "Labo-Lusion", stirring part: Homodisper 2.5 type). Further, as an active material, graphite (manufactured by Jiangxi Zishen Technology Co., Ltd., artificial graphite G1-C, average particle diameter D ５０The mixture was stirred at 2,000 rpm for 10 minutes using the disperser, and then degassed at 0.04 atmospheres for 5 minutes to obtain a lithium-ion battery electrode slurry with a solids concentration of 50% by mass. The prepared lithium-ion battery electrode slurry was applied to a copper foil (rolled copper foil, manufactured by UACJ Corporation, thickness 10 μm) using a 280 μm gap applicator, and then dried in an oven at 120° C. for 10 minutes to obtain a lithium-ion battery electrode (negative electrode) having a 150 μm-thick active material layer on a current collector (copper foil).

[0078] [Evaluation of electrode formability] The surface of the active material layer of the lithium ion battery electrode prepared by the above method was visually observed, and an active material layer with no cracks was rated as "A", and an active material layer with cracks was rated as "F". Note that Comparative Example 3 was rated as "ineligible for evaluation" because, as mentioned above, no water-soluble polymer was obtained.

[0079] [Method for Measuring the Amount of Residual Monomer] The aqueous binder solutions for lithium ion batteries obtained in the Examples and Comparative Examples were used as samples, and the amount of residual monomer was measured. １ H nuclear magnetic resonance spectroscopy was carried out to quantify the amount of residual monomer in the aqueous solution. In Table 1, the amount of residual monomer is shown as the ratio (mol %) of the amount of residual monomer to the total amount of monomers used as raw materials for the water-soluble polymer. The nuclear magnetic resonance (NMR) spectrometer used had the following specifications: Apparatus: Product name "Biospin Avance 500" manufactured by Bruker Corp. Deuterated solvent: Heavy water (D ２ O)

[0080]

[0081] The details of the monomers in Table 1 are as follows: DEGV: Diethylene glycol monovinyl ether HEVE: 2-hydroxyethyl vinyl ether ATBSNa: 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt SMAS: Sodium methallyl sulfonate AM: Acrylamide SA: Sodium acrylate

[0082] From Table 1, it can be seen that the aqueous binder solutions for lithium ion batteries of the present embodiment obtained in Examples 1 and 2 are capable of forming electrodes, and the generation of residual monomers during the synthesis of the water-soluble polymer is suppressed.

Claims

1. A binder for lithium ion batteries containing a water-soluble polymer (A), wherein the water-soluble polymer (A) contains structural units derived from a hydroxyl group-containing vinyl ether (a) and structural units derived from an ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof, the content of the structural units derived from the hydroxyl group-containing vinyl ether (a) in the water-soluble polymer (A) is 5 to 80 mol % of all structural units of the water-soluble polymer (A), and the content of the structural units derived from the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof in the water-soluble polymer (A) is 5 to 30 mol % of all structural units of the water-soluble polymer (A).

2. The binder for lithium ion batteries according to claim 1, wherein the water-soluble polymer (A) further contains structural units derived from a (meth)acrylamide group-containing compound (c) in an amount of 10 to 90 mol % based on the total structural units of the water-soluble polymer (A).

3. The binder for lithium ion batteries according to claim 1 or 2, wherein the hydroxyl group-containing vinyl ether (a) is a vinyl ether having a hydroxyalkyl group having 1 to 3 carbon atoms, or a vinyl ether having a hydroxypolyoxyalkylene chain containing two or more oxyalkylene units having 1 to 3 carbon atoms.

4. The binder for lithium ion batteries according to claim 1 or 2, wherein the ethylenically unsaturated monomer (b) having a sulfonic acid group or a salt thereof is a compound having a sulfonic acid group or a salt thereof and a (meth)acryloyl group.

5. An aqueous binder solution for lithium ion batteries, comprising the binder for lithium ion batteries according to claim 1 or 2 and water.

6. The aqueous binder solution for lithium ion batteries according to claim 5, wherein the total content of the water-soluble polymer (A) and water is 80 to 100 mass % of the total amount of the aqueous binder solution for lithium ion batteries.

7. An electrode slurry for a lithium ion battery, comprising the binder for a lithium ion battery according to claim 1 or 2, an active material, and water.

8. An electrode for a lithium ion battery, comprising the binder for a lithium ion battery according to claim 1 or 2 and an active material.

9. A lithium ion battery comprising the lithium ion battery electrode according to claim 8.

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